THIS INVENTION relates to transducers or pickups for stringed musical instruments whose output is intended to be amplified. In particular, the invention provides an improved noise cancellation pickup.
The invention will be described by way of example with reference to the musical instrument to which the pickups are fitted as being electric guitars. It should be appreciated that this is by way of example only and that instruments other than guitars may also be fitted with pickups according to the invention.
Electric guitars typically have at least four strings which when vibrated produce an output for amplification. The vibration of the strings is converted to electrical signals by pickups. The frequency of the electrical signals produced by the pickups corresponds to the frequency of vibration of the strings.
Pickups typically consist of a single bar magnet within a coil or a plurality of permanent magnets with a coil. The strings of the guitar are made of a magnetically permeable material typically a ferromagnetic material and the magnetic lines of flux developed by the permanent magnets are intercepted by the vibrating strings. This causes variations in the field pattern and a varying current is caused to flow in the coils. The frequency of the current corresponds to the frequency of vibration of the strings.
The coils, as well as being influenced by vibration of the strings also are subjected to noise. Noise is produced by lighting, electric motors and appliances and other sources. This noise, or hum adversely affects the quality of the sound reproduced by the pickups. The fundamental frequency of the electrical supply voltage, typically 50 Hz or 60 Hz, is converted into an audible hum in the amplifying equipment.
Many attempts have been made at ways of reducing or eliminating this noise but these attempts have introduced other undesirable effects.
Leo Fender in the 1940s was responsible for developing a single coil pickup. His design was particularly noise prone and equated basically to a long antenna for extraneous noise such as 50 Hz or 60 Hz hum and buzz caused by electric motors, lighting and other electrical appliances.
U.S. Pat. No. 4,442,749 issued to DiMarzio discloses one such earlier attempt at reducing noise. DiMarzio disclosed an electrical pickup device for stringed instruments. The device had a pair of superimposed coaxial bobbins each axially wound with a coil having its axis perpendicular to the strings of the instrument. An integral shield of magnetic material was present and had a base disposed between the two bobbins perpendicular to the coil axis and two side walls extend upwardly and perpendicularly from the base to at least immediately below the top face of the upper bobbin. A plurality of rod-like permanent magnets extended through the upper and lower coils. Thus, a plurality of magnets common to both coils were arranged within the coils.
The shield extended around three sides of the pickup coil. The shield was not particularly effective and allowed the magnetic field in the pickup coil to influence the lower noise reducing coil to affect the inductance of the lower coil and the electrical signals induced into that coil. The tonal structure of the pickup as a whole was adversely affected when the inductance was reduced below an acceptable level and one way to remedy this was to overwind the coils.
DiMarzio in a first device employed magnetic pole pieces common to both coils and this prohibits attaining a suitable overall inductance value because of inductance cancellation between the two coils.
DiMarzio in a second embodiment discloses a pickup having an upper coil with a plurality of magnetic pole pieces arranged within it. A lower noise cancelling coil is also shown. A channel shaped member receives the upper coil. Although the channel member extends around the upper coil, the coils are not effectively magnetically and inductively decoupled from one another. By doing this noise cancellation is achieved at the expense of tone quality.
An attempt at noise cancellation in pickup design was also made by Seymour Duncan. His design used full length Alnico V magnets which extended vertically through two coils. Like the DiMarzio design, the Duncan design also caused inductance and signal cancellation. Duncan did not employ any kind of magnetic barrier to separate the upper and lower coils.
A company known as EMG produced a pickup design known as Strat Vintage or SV. EMG employed full length magnets which extended through both an upper and a lower coil. Each coil was separately buffered into a two input differential operational amplifier but the inductance was less than 2.5 H since the inductance of the top half coil was 0.8 H. The lower coil was of similar inductance.